It is well known in the art to provide transmission arrangements for agricultural, construction, or industrial vehicles or machines that allow for rapid changes in the direction of movement of the vehicles or machines. This allows such vehicles or machines that must repeatedly change travel direction to do so without extensive braking and gear changes by the operator. As an example, a loader loading a dump truck must accelerate forward into a pile in order to fill its loader bucket, then reverse out of the pile, move forward to the dump truck, reverse away from the dump truck, and then accelerate again toward the pile. This cycle may be repeated several times for each loading event. Using a conventional geared transmission and service brakes, each cycle may involve four or more applications of the service brakes, four or more depressions of the clutch, and four or more gear changes, all while attending to the operation of the loader bucket and to the surrounding environment.
To minimize the amount of necessary coordination, a known type of transmission arrangement for agricultural, construction, or industrial vehicles or machines that allow for rapid changes in the direction of movement is a power-shuttle transmission. This type of transmission has two separate clutches that are typically operated with a single lever. One clutch engages a forward gear train, and the other clutch engages a reverse gear train. When transitioning from forward to reverse, or vice versa, the operator needs only move the shuttle lever from one direction to the other. In response, one clutch disengages and the other engages. The engaging clutch operates to absorb the inertia or kinetic energy of the vehicle, acting essentially as a brake until the vehicle reaches zero velocity, before further operating to engage its gear train to cause the vehicle to move in the other direction.
Constant use of the power-shuttle transmission clutches to not only engage opposing direction gear trains, but also to absorb the inertia or kinetic energy of the heavy moving vehicle or machine, often results in a large amount of wear and stress on the clutches, so that the clutches must be greatly overdesigned. Furthermore, even in an overdesigned condition, the repeated engagement of the power-shuttle transmission clutches against the inertia of the heavy moving vehicle or machine generates a tremendous amount of heat, which must be removed from the transmission and dissipated to the surrounding environment. Additionally, as the inertia or kinetic energy of the moving vehicle or machine must be absorbed against the force of the vehicle engine, use of the conventional power-shuttle transmission results in higher fuel usage and accompanying costs, as well as increased vehicle or machine tailpipe emissions.
What is needed in the art, therefore, is a way to take advantage of the power and convenience of a power-shuttle transmission, while minimizing the amount of wear and tear experienced by the shuttle clutches and/or wear experienced by the service brakes, and further minimizing the heat load generated within the power-shuttle transmission.